Chromia-containing refractories, such as chromia-alumina bricks, find advantageous use in coal gasifiers, municipal incinerators and applications where resistance to aggressive, fluid slag is important. Some chromia-alumina refractories are comprised of pigment-grade chromic oxide and a fused chromia-alumina grain. The pigment-grade chromic oxide and chromia-alumina grain are typically mixed, pressed into shapes, such as bricks, and then fired. Historically, chromia-alumina refractories with high chromic oxide contents have relatively low strength when measured at both room temperature and at operating temperatures. The strength of a chromia-containing refractory tends to decrease as the chromia content in the refractory increases. In this respect, the reduction in strength is particularly noticeable in chromia-containing refractories having chromic oxide contents in excess of 80% by weight, as measured by chemical analysis. One of the main reasons for the low strength is the lack of bonding between the fused high-chromia grains and the powdered, pigment-grade chromic oxide.
In addition, chromia-alumina refractories are prone to penetration of slags during service. The penetrated area is altered and is subject to thermal spalling. Thus, it would be desirable to increase the strength of chromia-alumina refractories, and to reduce the permeability of these refractories to inhibit slag penetration.
Efforts have been made to improve the strength and to decrease the permeability of these products. Attempts to increase the bond strength between the chromia-alumina fused grain and the pigment-grade chromic oxide by increasing the firing temperature generally do not improve the products because of chrome vaporization that occurs at temperatures beyond the normal firing temperatures. Adjustments to the particle size distributions of the respective components of these compositions, which in theory could reduce permeability, is difficult because pigment-grade chromic oxides are only commercially available in a few sizes. Still further, pressing a chromia-alumina refractory composition to very high densities helps to improve physical properties, but high-chromia refractories are very sensitive to cracking under high forming pressures (a phenomenon known as “pressure cracking”).
The present invention provides a chromia-containing refractory having improved strength, lower permeability and increased resistance to slag penetration, as compared to chromia-containing refractories known heretofore.